This invention relates to a light alloy piston for internal combustion engines, which piston comprises a top portion including a piston head formed with a combustion chamber recess and the ring groove portion and also comprises a skirt with integrated piston pin bosses which are provided with a block support or trapezoidal support.
Under the mechanical loads which are due to the ignition pressure, the head of light-alloy pistons for internal combustion engines is subjected to such a deformation in operation that under the load applied that the head arches upwardly over the piston pin bosses line a beam which is supported only in a small width and the ring grooves are arched too. Whereas the light-alloy piston can be designed to resist deformation so that its deformation will be keep within controllable limits, a support of the piston head for a proper transmission of force can be provided by piston pin bosses having various configurations, each of which will be optimum from the aspect of weight under certain boundary conditions. For a high ignition pressure loading, a block support for the piston pin bosses is preferred; in that case the piston pin bosses are straight on the inside and merge into the piston head without an undercut. A trapezoidal support is a modified block support and is used for very highly loaded pistons, particularly diesel pistons: in that case the piston pin bosses are beveled on the inside under an angle of inclination of 8 to 15 degrees and a correspondingly beveled connecting rod is employed. Owing to the partial overlap of the bearing surfaces a trapezoidal support will reduce the bending stress in the piston head and in the piston pin so that the permissible load will be increased by up to 15%. Even if the support for the piston pin bosses, i.e., the zone between the top apex of the bores in the piston pin bosses and the piston head, is designed to withstand high loads a maximum ignition pressure of 150 to 170 bars will subject those portions of the rim of the combustion chamber recess which are subjected to the highest loading by the ignition pressure to a loading which exceeds to the highest loading of about 30 N/mm.sup.2 which is permissible in a cast light-alloy piston having conventionally designed piston pin bosses. Besides, a conical flaring of that portion of the bores in the piston pin bosses which is adjacent to the connecting rod, or a necking of the piston in the vertical plane which contains those ends of the piston pin bosses which are adjacent to the connecting rod, or a provision of lateral recesses, so-called oil pockets, in the bearing surfaces of the bores in the piston pin bosses, will result in an increase of the stresses in those portions of the combustion chamber recess which are subjected to the highest loading by the ignition pressure.